Have you checked that you are actually accepting sound input on the line-in of the sound card? Bring up a mixer and check if that channel’s enabled for capture and the slider is up and any gain boost is turned on if needed.
Of course you should also check that the TV card is actually generating output. A connection an amplifier is useful here.
I have an analog TV card too and I can believe sound without picture, but picture without sound is harder because the sound is modulated along with the picture on the carrier.